Receivers in communication systems commonly incorporate one or more low noise amplifiers (LNAs) with suitable performance characteristics. For example single-tone desensitization for code division multiple access (CDMA) telephones generally specify a very high LNA third-order intercept point IIP3 in combination with low noise factor (NF), high gain, and low current consumption. Linearization techniques are commonly used to attain appropriate performance.
For example, feed-forward distortion cancellation has been used to attain a very high IIP3 for a complementary metal-oxide semiconductor (CMOS) low noise amplifier. Feed-forward distortion cancellation has not been widely adopted because of high sensitivity to mismatches between main and auxiliary gain stages and errors in input signal scaling.
A field-effect transistor can also be linearized by biasing at a gate-source voltage (VGS) at which a third-order derivative of the transistor DC transfer characteristic is zero. Resulting transistor third-order intercept point IIP3 peaks in a very narrow range of VGS, making the technique very sensitive to bias variations.
A derivative superposition (DS) method has been used to reduce third-order intercept point IIP3 sensitivity to bias. Derivative superposition uses two or more parallel field-effect transistors of different gate widths and gate biases to attain a composite DC transfer characteristic with an extended VGS range in which the third-order derivative is close to zero. However, the third-order intercept point IIP3 improvement attained is only modest at radio frequency, for example about 3 dB. Reducing source degeneration inductance and drain load impedance at a second-harmonic frequency of a composite input transistor allows an increase in third-order intercept point IIP3 by as much as 10 dB. One disadvantage of the derivative superposition technique is a resulting higher noise factor, for example in a range of about 0.6 dB.
The conventional derivative superposition method does not significantly increase third-order intercept point IIP3 at radio frequency due to the contribution of second-order nonlinearity to third-order intermodulation distortion (IMD3). In general, the vector of the second-order contribution is not collinear with the vector of the third-order contribution and, therefore, the contributions do not mutually cancel.